Wow! I'm SO glad to see you're going to spend some time there; I always feel so bad when I see people try to do a Disney trip in 3 days!
If it were me, I would spend 1 day at Animal Kingdom, 1-2 days at MGM, 2-3 days at Epcot, 2-3 days at MK, some time at the Water Parks, some time at Downtown Disney.
It will be your personal preferences on how much time you spend at each park, and it will also depend (sometimes) on the ages & make-up of your group that is going. Personally, being older, (43!), I enjoy Epcot, and could spend a week there, browsing the World Showcase. However, my active teenage sons LOVE MGM and the 2 big thrill-rides there.
I would start investigating what each park offers by visiting Deb's site at
www.allearsnet.com and then seeing what parks appeal to you most....!
Have fun!